Gate structure with additional oxide layer

ABSTRACT

A semiconductor structure and a method for forming the same are provided. The semiconductor structure includes a substrate and a gate structure formed over the substrate. The gate structure includes a gate dielectric layer formed over the substrate and a capping layer formed over the gate dielectric layer. The gate structure further includes a capping oxide layer formed over the capping layer and a work function metal layer formed over the capping oxide layer. The gate structure further includes a gate electrode layer formed over the work function metal layer.

BACKGROUND

Semiconductor devices are used in a variety of electronic applications, such as personal computers, cell phones, digital cameras, and other electronic equipment. Semiconductor devices are typically fabricated by sequentially depositing insulating or dielectric layers, conductive layers, and semiconductor layers of material over a semiconductor substrate, and patterning the various material layers using lithography to form circuit components and elements thereon.

However, although existing semiconductor manufacturing processes have generally been adequate for their intended purposes, as device scaling-down continues, they have not been entirely satisfactory in all respects.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It should be noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIGS. 1A to 1G are cross-sectional representations of various stages of forming a semiconductor structure in accordance with some embodiments.

FIGS. 2A to 2I are cross-sectional representations of various stages of forming a semiconductor structure in accordance with some embodiments.

FIG. 3 is a cross-sectional representation of a semiconductor structure 100 c in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Furthermore, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

Embodiments of semiconductor structures and methods for manufacturing the same are provided. The semiconductor structure includes a gate structure formed over a substrate, and the gate structure includes a gate dielectric layer, a capping layer formed over the gate dielectric layer, and a capping oxide layer formed over the capping layer in accordance with some embodiments. The capping oxide layer may be used as an oxygen source for the gate dielectric layer and as a barrier layer to prevent the metal formed over it from penetrating to the substrate.

FIGS. 1A to 1G are cross-sectional representations of various stages of forming a semiconductor structure 100 a in accordance with some embodiments. As shown in FIG. 1A, a substrate 102 is provided in accordance with some embodiments. Substrate 102 may be a semiconductor wafer such as a silicon wafer. Alternatively or additionally, substrate 102 may include elementary semiconductor materials, compound semiconductor materials, and/or alloy semiconductor materials. Examples of the elementary semiconductor materials may be, but are not limited to, crystal silicon, polycrystalline silicon, amorphous silicon, germanium, and/or diamond. Examples of the compound semiconductor materials may be, but are not limited to, silicon carbide, gallium arsenic, gallium phosphide, indium phosphide, indium arsenide, and/or indium antimonide. Examples of the alloy semiconductor materials may be, but are not limited to, SiGe, GaAsP, AlInAs, AlGaAs, GaInAs, GaInP, and/or GaInAsP.

An interfacial layer 104, a dummy gate dielectric layer 106, a dummy gate electrode layer 108 are formed over substrate 102, as shown in FIG. 1A in accordance with some embodiments. In some embodiments, interfacial layer 104 is made of SiO₂. Interfacial layer 104 may be formed by performing an atomic layer deposition (ALD) process, thermal oxidation process, UV-Ozone oxidation process, or chemical vapor deposition (CVD) process.

Dummy gate dielectric layer 106 is formed over interfacial layer 104 and is made of high-k dielectric materials, such as metal oxide in accordance with some embodiments. Examples of the high-k dielectric material include, but are not limited to, hafnium oxide (HfO₂), hafnium silicon oxide (HfSiO), hafnium tantalum oxide (HfTaO), hafnium titanium oxide (HfTiO), hafnium zirconium oxide (HfZrO), zirconium oxide, titanium oxide, aluminum oxide, hafnium dioxide-alumina (HfO₂—Al₂O₃) alloy, or other applicable dielectric materials. Dummy gate dielectric layer 106 may be formed by chemical vapor deposition (CVD), physical vapor deposition, (PVD), atomic layer deposition (ALD), spin-on coating, or other applicable processes.

After dummy gate dielectric layer 106 is formed, dummy gate electrode layer 108 is formed over dummy gate dielectric layer 106 in accordance with some embodiments. In some embodiments, dummy gate electrode layer 108 is made of polysilicon. Next, interfacial layer 104, dummy gate dielectric layer 106, and dummy gate electrode layer 108 are patterned to form a dummy gate structure 110, as shown in FIG. 1B in accordance with some embodiments.

Afterwards, sealing layers 112 are formed on the sidewalls of dummy gate structure 110, as shown in FIG. 1C in accordance with some embodiments. Sealing layer 112 may protect dummy gate structure 110 from damage or loss during subsequent processing and may also prevent oxidation during subsequent processing. In some embodiments, sealing layer 112 is made of silicon nitride, silicon oxide, silicon oxynitride, silicon carbide, or other applicable dielectric materials. Sealing layer 112 may include a single layer or multiple layers.

Spacers 114 are further formed on sealing layer 112 in accordance with some embodiments. In some embodiments, spacers 114 are made of silicon nitride, silicon oxide, silicon carbide, silicon oxynitride, or other applicable materials.

Source/drain structures 116 are formed in substrate 102 adjacent to dummy gate structure 110, as shown in FIG. 1C in accordance with some embodiments. In some embodiments, source/drain structures 116 are formed by using an implantation process or an epitaxial (epi) process. In some embodiments, source/drain structures 116 include Ge, SiGe, InAs, InGaAs, InSb, GaAs, GaSb, InAlP, InP, or the like.

After source/drain structures 116 are formed in substrate 102, a contact etch stop layer (CESL) 118 is formed over substrate 102, and an interlayer dielectric layer 120 is formed over contact etch stop layer 118, as shown in FIG. 1D in accordance with some embodiments. As shown in FIG. 1D, contact etch stop layer 118 is formed on the sidewalls of spacers 114 and the top surface of source/drain structures 116 in accordance with some embodiments. In some embodiments, contact etch stop layer 118 is made of silicon nitride, silicon oxynitride, and/or other applicable materials. Contact etch stop layer 118 may be formed by chemical vapor deposition (CVD), physical vapor deposition, (PVD), atomic layer deposition (ALD), spin-on coating, or other applicable processes. In some embodiments, interlayer dielectric layer 120 is made of silicon oxide, silicon nitride, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), and/or other applicable dielectric materials. Interlayer dielectric layer 120 may be formed by chemical vapor deposition (CVD), physical vapor deposition, (PVD), atomic layer deposition (ALD), spin-on coating, or other applicable processes.

After interlayer dielectric layer 120 is formed, dummy gate structure 110 is removed to form a trench 122 between spacers 114, as shown in FIG. 1E in accordance with some embodiments. Afterwards, a metal gate structure 124 is formed in trench 122 in accordance with some embodiments.

More specifically, metal gate structure 124 includes an interfacial layer 126, a gate dielectric layer 128, a capping layer 130, a capping oxide layer 132, a work functional metal layer 134, and a metal gate electrode layer 136 in accordance with some embodiments. As shown in FIG. 1F, interfacial layer 126 is formed over the top surface of substrate 102 which is exposed by trench 122. Interfacial layer 126 is formed between substrate 102 and gate dielectric layer 128, so that the adhesion of gate dielectric layer 128 can be improved. In some embodiments, interfacial layer 126 is made of SiO₂. In some embodiments, interfacial layer 126 has a thickness in a range from about 5 Å to about 20 Å. In some embodiments, interfacial layer 126 is formed by performing an atomic layer deposition (ALD) process, thermal oxidation process, UV-Ozone oxidation process, or chemical vapor deposition (CVD) process.

After interfacial layer 126 is formed, gate dielectric layer 128 is formed over interfacial layer 126, as shown in FIG. 1F in accordance with some embodiments. In some embodiments, gate dielectric layer 128 is made of a high k dielectric material, such as metal oxide. In some embodiments, gate dielectric layer 128 is made of hafnium oxide (HfO₂), hafnium silicon oxide (HfSiO), hafnium tantalum oxide (HfTaO), hafnium titanium oxide (HfTiO), hafnium zirconium oxide (HfZrO), zirconium oxide, titanium oxide, aluminum oxide, hafnium dioxide-alumina (HfO₂—Al₂O₃) alloy, or other applicable dielectric materials. In some embodiments, gate dielectric layer 128 has a thickness in a range from about 10 Å to about 30 Å. Gate dielectric layer 128 may be formed by chemical vapor deposition (CVD), physical vapor deposition, (PVD), atomic layer deposition (ALD), spin-on coating, or other applicable processes.

After gate dielectric layer 128 is formed, capping layer 130 is formed over gate dielectric layer 128, as shown in FIG. 1F in accordance with some embodiments. Capping layer 130 is configured to prevent the metal formed over it from penetrating into the channel region in substrate 102 below metal gate structure 124. In some embodiments, capping layer 130 has a thickness T₁ in a range from about 5 Å to about 100 Å. If capping layer 130 is not thick enough, the metal formed over capping layer 130 may still penetrate through it and enter into substrate 102. On the other hand, if capping layer 130 is too thick, too much oxygen in gate dielectric layer 128 may be drawn by capping layer 130, resulting in a great amount of oxygen vacancy in gate dielectric layer 128.

In some embodiments, capping layer 130 is made of metal nitride. Examples of materials used to form capping layer 130 may include, but are not limited to, TiN, TaN, and WN. Capping layer 130 may be formed by chemical vapor deposition (CVD), physical vapor deposition, (PVD), atomic layer deposition (ALD), spin-on coating, or other applicable processes. In some embodiments, capping layer 130 is formed by performing a thermal CVD process at a temperature in a range from about 200° C. to about 500° C. If the temperature of the process is too high, the resistance of the resulting capping layer 130 may be increased.

After capping layer 130 is formed, a capping oxide layer 132 is formed over capping layer 130, as shown in FIG. 1G in accordance with some embodiments. Capping oxide layer 132 is formed over capping layer 130 to further insure that the metal formed thereover will not penetrate (e.g. diffuse) into substrate 102. That is, capping oxide layer 132 is also configured to prevent the metal from penetrating or diffusing into substrate 102.

In some embodiments, capping oxide layer 132 is made of metal oxynitride. In some embodiments, capping oxide layer 132 is made of MN_(x)O_(y), and M is Ti, Ta, or W, N is nitrogen, O is oxygen, x is in a range from about 4 to about 9, y is in a range from about 5 to about 9. In some embodiments, a ratio of x to y is in a range of about 0.4 to about 1.8. If the ratio of x to y is too large, there may not be enough oxygen to prevent the metal formed thereon from penetrating. On the other hand, if the ratio of x to y is too small, it may contain too much oxygen resulting in the regrowth of the interfacial layer. In some embodiments, capping oxide layer 130 is made of TiN, and capping oxide layer 132 is made of TiN_(x)O_(y).

In some embodiments, capping oxide layer 132 is made of TiO_(y). In some embodiments, y is in a range from about 5 to about 9.

In some embodiments, capping oxide layer 132 is formed by oxidizing capping layer 130. In some embodiments, capping oxide layer 132 is formed by performing an annealing process. In some embodiments, capping oxide layer 132 is formed by oxidizing capping layer 130 at a temperature in a range from about 600° C. to about 1000° C. As described previously, capping oxide layer 132 is configured to prevent the metal formed over it from diffusing through. Therefore, if the temperature for forming capping oxide layer 132 is not high enough, the resulting capping oxide layer 132 may not be thick enough to stop the metal. On the other hand, if the temperature for forming capping oxide layer 132 is too high, the resulting capping oxide layer 132 may be too thick, resulting in undermining the performance of metal gate structure 124. In addition, interfacial layer 126 may also become thicker and the gate dielectric layer 128 may be crystallized due to the high temperature.

In some embodiments, capping oxide layer 132 has a thickness T₂ in a range from about 1 Å to about 50 Å. Since oxide is a relatively rigid material, capping oxide layer 132 does not need to be very thick to block the metal from penetrating into it. However, if capping oxide layer 132 is too thin, it may not be able to stop the metal completely. On the other hand, if capping oxide layer 132 is too thick, the capacitance of the resulting metal gate structure 124 may be increased and its performance may therefore be undermined.

In some embodiments, the thickness T₁ of capping layer 130 is greater than the thickness T₂ of capping oxide layer 132. In some embodiments, the sum of thickness T₁ of capping layer 130 and thickness T₂ of capping oxide layer 132 is in a range from about 6 Å to about 150 Å. As described previously, since capping layer 130 and capping oxide layer 132 are configured to block the metal formed over them from penetrating through, capping layer 130 and capping oxide layer 132 should be thick enough. However, if capping layer 130 and capping oxide layer 132 are too thick, the performance of the resulting metal gate structure 124 may be undermined.

After capping oxide layer 132 is formed, work function metal layer 134 is formed over capping oxide layer 132, and metal gate electrode layer 136 is formed over work function metal layer 134, as shown in FIG. 1G in accordance with some embodiments. Work function metal layer 134 is tuned to have a proper work function. In some embodiments, work function metal layer 134 is made of metal nitride. For example, if a P-type work function metal (P-metal) for a PMOS device is desired, TiN, WN, or W may be used. On the other hand, if an N-type work function metal (N-metal) for NMOS devices is desired, TiAl, TiAlN, or TaCN, may be used.

In some embodiments, metal gate electrode layer 136 is made of a conductive material, such as a metal. Examples of the conductive materials used to form metal gate electrode 136 may include, but are not limited to, aluminum, copper, tungsten, titanium, tantalum, or other applicable materials. As described previously, in subsequent processes, the metal of metal gate electrode layer 136 may diffuse toward the layers formed below. However, since capping layer 130 and capping oxide layer 132 are formed below metal gate electrode layer 136, the metal of metal gate electrode layer 136 can be blocked by them and will not penetrate through gate dielectric layer 128 and enter into substrate 102.

FIGS. 2A to 2I are cross-sectional representations of various stages of forming a semiconductor structure 100 b in accordance with some embodiments. Although the sequence of forming each element in semiconductor structure 100 b may be different from that in semiconductor structure 100 a shown in FIGS. 1A to 1G and described previously, the materials and processes being used are similar or the same. Therefore, those materials and processes are not repeated herein.

As shown in FIG. 2A, interfacial layer 126 is formed over substrate 102, and gate dielectric layer 128 is formed over interfacial layer 126 in accordance with some embodiments. As described previously, interfacial layer 126 is formed between substrate 102 and gate dielectric layer 128, so that the adhesion of gate dielectric layer 128 can be improved.

After gate dielectric layer 128 is formed, capping layer 130 is formed over gate dielectric layer 128, as shown in FIG. 2B in accordance with some embodiments. As described previously, capping layer 130 is configured to prevent the metal formed over it from penetrating into substrate 102. In some embodiments, the thickness T₁ of capping layer 130 is in a range from about 10 Å to about 50 Å.

After capping layer 130 is formed, capping oxide layer 132 is formed over capping layer 130, as shown in FIG. 2C in accordance with some embodiments. As described previously, capping oxide layer 132 is configured to prevent the metal formed over it from diffusing into substrate 102. In some embodiments, capping oxide layer 132 has a thickness T₂ in a range from about 5 Å to about 10 Å.

After capping oxide layer 132 is formed, dummy gate electrode layer 108 is formed over capping oxide layer 132, as shown in FIG. 2D in accordance with some embodiments. Afterwards, dummy gate electrode layer 108, capping oxide layer 132, capping layer 130, gate dielectric layer 128, and interfacial layer 126 are patterned to form a dummy gate structure 110′, as shown in FIG. 2E in accordance with some embodiments.

After dummy gate structure 110′ is formed, sealing layers 112 are formed over the sidewalls of dummy gate structure 110′, and spacers 114 are formed over sealing layers 112, as shown in FIG. 2F in accordance with some embodiments. In addition, source/drain structures 116 are formed adjacent to dummy gate structure 110′ in substrate 102.

Next, contact etch stop layer 118 is formed over substrate 102, and interlayer dielectric layer 120 is formed over contact etch stop layer 118, as shown in FIG. 2G in accordance with some embodiments. Afterwards, dummy gate electrode layer 108 is removed to form trench 122′ between spacers 114, as shown in FIG. 2H in accordance with some embodiments.

After trench 122′ is formed, work function metal layer 134 is formed in trench 122′, and metal gate electrode layer 136 is formed over work function metal layer 134, as shown in FIG. 2I in accordance with some embodiments. As shown in FIG. 2I, semiconductor structure 100 b includes a metal gate structure 124′, which further includes interfacial layer 126, gate dielectric layer 128, capping layer 130, capping oxide layer 132, work functional layer 134, and metal gate electrode layer 136. In addition, interfacial layer 126, gate dielectric layer 128, capping layer 130, and capping oxide layer 132 in metal gate structure 124′ are formed before dummy gate electrode layer 108 is formed in accordance with some embodiments. That is, gate dielectric layer 128, capping layer 130, and capping oxide layer 132 may be formed after or before dummy gate electrode layer 108 is removed, as shown in FIGS. 1A to 1G and FIGS. 2A to 2I, and the scope of the disclosure is not intended to be limited.

FIG. 3 is a cross-sectional representation of a semiconductor structure 100 c in accordance with some embodiments. Semiconductor structure 100 c includes a first metal gate structure 124 a and a second metal gate structure 124 b formed over substrate 102, and first metal gate structure 124 a and second metal gate structure 124 b are separated by an isolation structure 301 in accordance with some embodiments.

As shown in FIG. 3, first metal gate structure 124 a includes a first interfacial layer 126 a, a first gate dielectric layer 128 a, a first capping layer 130 a, a first capping oxide layer 132 a, a first work function metal layer 134 a, and a first metal gate electrode layer 136 a in accordance with some embodiments. First interfacial layer 126 a is formed on the top surface of substrate 102 and may be formed by the materials and processes used to form interfacial layer 126 described previously. First gate dielectric layer 128 a is formed on first interfacial layer 126 a and may be formed by the materials and processes used to form gate dielectric layer 128 described previously.

First capping layer 130 a is formed on first gate dielectric layer 128 a and may be formed by the materials and processes used to form capping layer 130 described previously. First capping oxide layer 132 a is formed on first capping layer 130 a and may be formed by the materials and processes used to form capping oxide layer 132 described previously. First work function metal layer 134 a is formed on first capping oxide layer 132 a and may be formed by the materials and processes used to form work function metal layer 134 described previously. First metal gate electrode layer 136 a is formed on first work function metal layer 134 a and may be formed by the materials and processes used to form metal gate electrode layer 136 described previously.

In addition, second metal gate structure 124 b includes a second interfacial layer 126 b, a second gate dielectric layer 128 b, a second capping layer 130 b, a second capping oxide layer 132 b, a second work function metal layer 134 b, and a second metal gate electrode layer 136 b in accordance with some embodiments. Second interfacial layer 126 b is formed on the top surface of substrate 102 and may be formed by the materials and processes used to form interfacial layer 126 described previously. Second gate dielectric layer 128 b is formed on second interfacial layer 126 b and may be formed by the materials and processes used to form gate dielectric layer 128 described previously.

Second capping layer 130 b is formed on second gate dielectric layer 128 b and may be formed by the materials and processes used to form capping layer 130 described previously. Second capping oxide layer 132 b is formed on second capping layer 130 b and may be formed by the materials and processes used to form capping oxide layer 132 described previously. Second work function metal layer 134 b is formed on second capping oxide layer 132 b and may be formed by the materials and processes used to form work function metal layer 134 described previously. Second metal gate electrode layer 136 b is formed on second work function metal layer 134 b and may be formed by the materials and processes used to form metal gate electrode layer 136 described previously.

Furthermore, sealing layers 112 are formed on the sidewalls of first metal gate structure 124 a and second metal gate structure 124 b, and spacers 114 are formed over sealing layer 112 in accordance with some embodiments. In addition, source/drain structures 116 are formed adjacent to first metal gate structure 124 a and second metal gate structure 124 b in substrate 102. Contact etch stop layer 118 is formed on the sidewalls of spacers 114 and over source/drain structures 116 over substrate 102, and interlayer dielectric layer 120 is formed over contact etch stop layer 118.

In some embodiments, first metal gate structure 124 a is used in a p-type metal-oxide-semiconductor (PMOS) transistor and second metal gate structure 124 b is used in an n-type metal-oxide-semiconductor (NMOS) transistor. In addition, first work function metal layer 134 a and second work function metal layer 134 b may be used to control the electrical type of transistors. Accordingly, first work function metal layer 134 a and second work function metal layer 134 b are made of different materials in accordance with some embodiments. For example, first work function metal layer 134 a is made of a P-type metal, such as TiN, WN, or W, and second work function metal layer 134 b is made of an N-type metal, such as TiAl, TiAlN, or TaCN.

In some embodiments, first capping layer 130 a and second capping layer 130 b are made of the same material and first capping oxide layer 132 a and second capping oxide layer are made of the same material. As described previously, first capping layer 130 a, second capping layer 130 b, first capping oxide layer 132 a, and second capping oxide layer 132 b are configured to prevent the metal formed over them (e.g. the metal in first metal gate electrode layer 136 a and second metal gate electrode layer 136 b) from diffusing into substrate 102. That is, the electrical type of transistors are not controlled by the materials used to form first capping layer 130 a, second capping layer 130 b, first capping oxide layer 132 a, and second capping oxide layer 132 b.

Accordingly, even if first metal gate structure 124 a is used in a PMOS transistor and second metal gate structure 124 b is used in an NMOS transistor, the material used to form first capping layer 130 a and second capping layer 130 b may still be the same. In some embodiments, first capping layer 130 a and second capping layer 130 b are made of metal nitride, and first capping oxide layer 132 a and second capping oxide layer 132 b are made of metal oxynitride.

In some embodiments, first capping layer 130 a and first work function metal layer 134 a are made of the same material, and second capping layer 130 b and second work function metal layer 134 b are made of different materials. In some embodiments, first capping layer 130 a and second capping layer 130 b are respectively thicker than first capping oxide layer 132 a and second capping oxide layer 132 b. In some embodiments, first capping oxide layer 132 a and second capping oxide layer 132 b have equal thickness in a range from about 5 Å to about 10 Å. In some embodiments, the sum of the thicknesses of first capping layer 130 a and first capping oxide layer 132 a is equal to the sum of the thicknesses of second capping layer 130 b and second capping oxide layer 132 b and is in a range from about 15 Å to about 60 Å.

As shown in FIGS. 1A to 3 and described previously, capping oxide layer 132 (or capping oxide layer 132 a or 132 b) is formed over capping layer 130 (or capping layer 130 a or 130 b). Capping oxide layer 132 is configured to act as an oxygen source for gate dielectric layer 128 formed below and to prevent the metal formed over it from diffusing into substrate 102.

More specifically, gate dielectric layer 128 tends to lose its oxygen during subsequent manufacturing processes, resulting in forming oxygen vacancies in gate dielectric layer 128. The oxygen vacancy in gate dielectric layer 128 may lead to high threshold voltage of the semiconductor structure, and the resulting devices may have poor uniformity. However, when capping oxide layer 132 is formed on capping layer 130 over gate dielectric layer 128, capping oxide layer 132 may be seen as an oxygen source to provide oxygen to gate dielectric layer 128. Therefore, the oxygen vacancy in gate dielectric layer 128 can be reduced and the uniformity of the resulting devices, including semiconductor structures 100 a, 100 b, and 100 c, can be improved.

Furthermore, a channel is formed in substrate 102 below metal gate structure 124 (or metal gate structure 124′, 124 a, or 124 b). The metal formed over the channel, such as the metal in gate electrode layer 136, tend to penetrate through the gate structure and enter into the channel. In this case, substrate spiking occurs, and the performance of the channel may be reduced. However, since capping oxide layer 132 is formed over capping layer 130, capping oxide layer 132 can be used as a barrier to block the metal over it. That is, capping oxide layer 132 may prevent the metal formed over it from diffusing through it. Since the metal formed over substrate 102 cannot penetrate gate dielectric layer 128 and enter into substrate 102, the performance of semiconductor structure 100 a (or semiconductor structures 100 b and 100 c) can be improved.

In addition, since capping oxide layer 132 is formed over capping layer 130 and is more rigid than capping layer 130, the thickness of capping layer 130 does not have to be very thick to block the metal. If capping layer 130 is too thick, it may also tend to draw the oxygen in gate dielectric layer 128 and the threshold voltage of the semiconductor structure may therefore increase. However, in the embodiments described above, capping oxide layer 132 is used to help capping layer 130 to block the metal over it. Therefore, capping layer 130 can be relatively thin. That is, by forming both capping layer 130 and capping oxide layer 132, the metal penetration can be prevented without resulting in the increase of the threshold voltage.

Moreover, capping oxide layer 132 is formed over capping layer 130, instead directly formed over gate dielectric layer 128. If an oxide layer, such as a TiN_(x)O_(y) layer, is formed directly over gate dielectric layer 128, the k value of the semiconductor structure may be increased, and the performance of the semiconductor structure may be reduced. However, in the embodiments described above, capping oxide layer 132 is formed over capping layer 130. Accordingly, the k value of gate dielectric layer 128 will not be affected, and the performance of semiconductor structure 100 a is not undermined.

Embodiments of semiconductor structures and methods for manufacturing the same are provided. The semiconductor structure includes a gate structure, and the gate structure includes a gate dielectric layer, a capping layer, a capping oxide layer, a work function metal layer, and a gate electrode layer. The capping oxide layer is formed over the capping layer to prevent the metal formed over it, such as the metal in the gate electrode layer, from penetrating the gate dielectric layer and entering the substrate below the gate structure. In addition, the capping oxide layer may also be used as an oxygen source to reduce the oxygen vacancy in the gate dielectric layer. Therefore, the performance of the semiconductor structure can be improved.

In some embodiments, a semiconductor structure is provided. The semiconductor structure includes a substrate and a gate structure formed over the substrate. The gate structure includes a gate dielectric layer formed over the substrate and a capping layer formed over the gate dielectric layer. The gate structure further includes a capping oxide layer formed over the capping layer and a work function metal layer formed over the capping oxide layer. The gate structure further includes a gate electrode layer formed over the work function metal layer.

In some embodiments, a semiconductor structure is provided. The semiconductor structure includes a substrate and a first gate structure formed over the substrate. The first gate structure includes a first gate dielectric layer formed over the substrate and a first capping layer formed over the first gate dielectric layer. The first gate structure further includes a first capping oxide layer formed over the first capping layer and a first work function metal layer formed over the first capping oxide layer. The first gate structure further includes a first gate electrode layer formed over the first work function metal layer. The semiconductor structure further includes a second gate structure formed over the substrate. The second gate structure includes a second gate dielectric layer formed over the substrate and a second capping layer formed over the second gate dielectric layer. The second gate structure further includes a second capping oxide layer formed over the second capping layer and a second work function metal layer formed over the second capping oxide layer. The second gate structure further includes a second gate electrode layer formed over the second work function metal layer. In addition, the first capping layer and the second capping layer are made of the same material, and the first work function metal layer and the second work function metal are made of different materials.

In some embodiments, a method for manufacturing a semiconductor structure is provided. The method for manufacturing a semiconductor structure includes forming a gate dielectric layer over a substrate and forming a capping layer over the gate dielectric layer. The method for manufacturing a semiconductor structure further includes forming a capping oxide layer over the capping layer and forming a work function metal layer over the capping oxide layer. The method for manufacturing a semiconductor structure further includes forming a gate electrode layer over the work function metal layer. In addition, the gate electrode layer is made of metal, and the capping oxide layer is configured to prevent the metal of the gate electrode layer from diffusing into the substrate.

The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

What is claimed is:
 1. A semiconductor structure, comprising: a substrate; and a first gate structure formed over the substrate, wherein the first gate structure comprises: a first gate dielectric layer formed over the substrate; a first capping layer formed over the first gate dielectric layer; a first capping oxide layer formed over the first capping layer, wherein the first capping oxide layer is made of TiN_(x)O_(y), and x:y is in a range from about 0.4 to about 1.8; a first work function metal layer formed over the first capping oxide layer; and a first gate electrode layer formed over the first work function metal layer; wherein a sidewall of the first capping oxide layer is surrounded by the first capping layer; a second gate structure formed over the substrate, wherein the second gate structure comprises: a second gate dielectric layer formed over the substrate; a second capping layer formed over the second gate dielectric layer; a second capping oxide layer formed over the second capping layer, wherein the second capping oxide layer is made of TiN_(x)O_(y), and x:y is in a range from about 0.4 to about 1.8; and a second work function metal layer formed over the second capping oxide layer, wherein the second capping oxide layer and the first capping oxide layer are made of the same material wherein the first work function metal layer is made of a P-type metal, and the second work function metal layer is made of an N-type metal.
 2. The semiconductor structure as claimed in claim 1, wherein the first capping layer is made of TiN.
 3. The semiconductor structure as claimed in claim 1, wherein the first capping oxide layer has a thickness in a range from about 1 Å to about 50 Å.
 4. The semiconductor structure as claimed in claim 1, wherein the first capping layer has a thickness in a range from about 5 Å to about 100 Å.
 5. The semiconductor structure as claimed in claim 1, wherein a sum of a thickness of the first capping layer and a thickness of the first capping oxide layer is in a range from about 6 Å to about 150 Å.
 6. A semiconductor structure, comprising: a substrate; a first gate structure formed over the substrate, wherein the first gate structure comprises: a first gate dielectric layer formed over the substrate; a first capping layer formed over the first gate dielectric layer; a first capping oxide layer formed over the first capping layer; a first work function metal layer formed over the first capping oxide layer; and a first gate electrode layer formed over the first work function metal layer, wherein a sidewall of the first capping oxide layer is surrounded by the first capping layer; and a second gate structure formed over the substrate, wherein the second gate structure comprises: a second gate dielectric layer formed over the substrate; a second capping layer formed over the second gate dielectric layer; a second capping oxide layer formed over the second capping layer; a second work function metal layer formed over the second capping oxide layer; and a second gate electrode layer formed over the second work function metal layer, wherein the first capping oxide layer and the second capping oxide layer are made of the same material, and the first work function metal layer and the second work function metal layer are made of different materials.
 7. The semiconductor structure as claimed in claim 6, wherein the first work function metal layer is made of an N-type metal, and the second work function metal layer is made of a P-type metal.
 8. The semiconductor structure as claimed in claim 6, wherein the first capping layer and the second capping layer are made of metal nitride, the first capping oxide layer and the second capping oxide layer are made of metal oxynitride.
 9. The semiconductor structure as claimed in claim 6, wherein a thickness of the first capping layer is greater than a thickness of the first capping oxide layer.
 10. The semiconductor structure as claimed in claim 6, wherein the first capping oxide layer has a thickness in a range from about 1 Å to about 50 Å.
 11. The semiconductor structure as claimed in claim 6, wherein the first capping layer has a thickness in a range from about 5 Å to about 100 Å.
 12. The semiconductor structure as claimed in claim 6, further comprising: a first sealing layer formed over a sidewall of the first gate structure; and a second sealing layer formed over a sidewall of the second gate structure, wherein the first gate dielectric layer, the first capping layer, the first capping oxide layer and the first work function metal layer are sequentially formed over the first sealing layer, and top surfaces of the first gate dielectric layer, the first capping layer, the first capping oxide layer and the first work function metal layer are substantially level, and the second gate dielectric layer, the second capping layer, the second capping oxide layer and the second work function metal layer are sequentially formed over the second sealing layer, and top surfaces of the second gate dielectric layer, the second capping layer, the second capping oxide layer and the second work function metal layer are substantially level.
 13. The semiconductor structure as claimed in claim 6, wherein the sidewall of the first capping oxide layer is in direct contact with the first capping layer.
 14. A semiconductor structure, comprising: a substrate; a first gate structure formed over the substrate, wherein the first gate structure comprises: a first gate dielectric layer formed over the substrate; a first capping layer formed over the first gate dielectric layer; a first capping oxide layer formed over the first capping layer; and a first work function metal layer formed over the first capping oxide layer, wherein a top surface of the first capping layer is substantially level with a top surface of the first capping oxide layer; and a second gate structure formed over the substrate, wherein the second gate structure comprises: a second gate dielectric layer formed over the substrate; a second capping layer formed over the second gate dielectric layer; a second capping oxide layer formed over the second capping layer; and a second work function metal layer formed over the second capping oxide layer, wherein the first gate structure is a p-type metal-oxide-semiconductor transistor and the second gate structure is a n-type metal-oxide-semiconductor transistor, the first capping layer and the first work function metal layer are made of the same material and the second capping layer and the second work function metal layer are made of different materials.
 15. The semiconductor structure as claimed in claim 14, wherein the first work function metal layer is made of an P-type metal, and the second work function metal layer is made of a N-type metal.
 16. The semiconductor structure as claimed in claim 15, further comprising: a first gate electrode layer formed over the first work function metal layer; and a second gate electrode layer formed over the second work function metal layer.
 17. The semiconductor structure as claimed in claim 14, wherein the first work function metal layer and the second work function metal layer are made of different materials.
 18. The semiconductor structure as claimed in claim 14, wherein the first capping oxide layer has a thickness in a range from about 1 Å to about 50 Å.
 19. The semiconductor structure as claimed in claim 18, wherein the first capping layer has a thickness in a range from about 5 Å to about 100 Å.
 20. The semiconductor structure as claimed in claim 14, wherein a sum of a thickness of the first capping layer and a thickness of the first capping oxide layer is in a range from about 6 Å to about 150 Å. 